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Yan Jiang, lead author of the Nature Sustainability study and postdoctoral scholar at UC San Diego with a joint appointment at the School of Global Policy and Strategy and Scripps Institution of Oceanography.
A New Way to Forecast Drought Risk
Using nearly two decades of satellite data, Jiang and co-author Jennifer Burney of Stanford University measured how much of the world’s rainfall comes from land-based evaporation. They discovered that when more than about one-third of rainfall originates from land, croplands are significantly more vulnerable to drought, soil moisture loss and yield declines – likely because ocean-sourced systems tend to deliver heavier rainfall, while land-sourced systems tend to deliver less reliable showers, increasing the chance of water deficits during critical crop growth stages.
This insight provides a new way for farmers and policymakers to identify which regions are most at risk — and to plan accordingly.
“For farmers in areas that rely heavily on land-originating moisture — like parts of the Midwest or eastern Africa — local water availability becomes the deciding factor for crop success,” Jiang explained. “Changes in soil moisture or deforestation can have immediate, cascading impacts on yields.”
Two Global Hotspots: The U.S. Midwest and East Africa
The study highlights two striking hotspots of vulnerability: the U.S. Midwest and tropical East Africa.
In the Midwest, Jiang notes, droughts have become more frequent and intense in recent years — even in one of the world’s most productive and technologically advanced farming regions.
“Our findings suggest that the Midwest’s high reliance on land-sourced moisture, from surrounding soil and vegetation, could amplify droughts through what we call ‘rainfall feedback loops,’” Jiang said. “When the land dries out, it reduces evaporation, which in turn reduces future rainfall—creating a self-reinforcing drought cycle.”
Because this region is also a major supplier to global grain markets, disruptions there have ripple effects far beyond U.S. borders. Jiang suggests that Midwestern producers may need to pay closer attention to soil moisture management, irrigation efficiency and timing of planting to avoid compounding drought stress.
In contrast, East Africa faces a more precarious but still reversible situation. Rapid cropland expansion and loss of surrounding rainforests threaten to undermine the very moisture sources that sustain rainfall in the region.
“This creates a dangerous conflict,” Jiang said. “Farmers are clearing forests to grow more crops, but those forests help generate the rainfall that the crops depend on. If that moisture source disappears, local food security will be at greater risk.”
However, Jiang sees opportunity as well as risk:
“Eastern Africa is on the front line of change, but there is still time to act. Smarter land management — like conserving forests and restoring vegetation — can protect rainfall and sustain agricultural growth.”